It has been hypothesized that much of the matter in the universe is "dark," i.e., unseen. Studies have shown that galaxies in many galaxy clusters are moving faster with respect to one another than they would if visible stars constituted all their mass. The studies suggest that the galaxies are moving under the gravitational influence of unseen mass in considerable quantity.

Which of the following is an assumption underlying the passage above?

Measurements of the speed of moving galaxies are extremely unreliable.

The workings of gravitational forces are not particularly well understood.

The aggregate mass of visible stars in the galaxies mentioned above can be estimated with some confidence.

The general composition of unseen matter in the universe has been determined.

Without exception, the galaxies mentioned above move toward one another.